This invention relates to the fumigation of grain and other commodities in bulk-storage compartments, and has particular reference to the fumigation of grain in the holds of seagoing vessels or other large containers with fumigants that produce toxic gases for permeating the body of grain and killing pests, primarily insects but sometime rodents as well.
In the shipment of grain and other commodities, it is a common practice to fumigate each storage compartment, or hold, of a ship by placing a quantity of gas-producing fumigant in the hold with the grain and sealing the hold so that toxic gas produced by the fumigant is confined in the hold for contact with the pests during shipment. The object is to produce lethal concentrations of the toxic gas throughout the body of grain in the compartment, and to maintain that concentration for the period of time necessary to kill all insects in the grain.
A popular type of fumigant that has been in use for many years is metallic phosphide, such as aluminum phosphide which decomposes in the presence of water to form hydrogen phosphide, or phosphine, a toxic gas that has been very successfully used as a fumigant. Reference is made to U.S. Pat. Nos. 2,826,486, 2,826,527 and 3,132,067 for details regarding fumigants of this general type, in which the fumigant in solid form, usually tablets or pellets, is activated by atmospheric moisture to produce the toxic gas, leaving only a residue of decomposed and usually harmless powder.
Phosphine is poisonous not only to insects and rodents but also to humans, so great care is exercised in its handling, and workers (longshoremen) usually will not enter a compartment after fumigation is commenced. For this reason, the usual practice has been to place the fumigant in the grain after a hold has been filled, or substantially filled, by inserting it under the surface of the grain with probes, or by laying it on top of the grain, sometimes in compartmented "blankets" or bags of moisture- and gas-pervious material. These bags or blankets facilitate handling of the gas producing solid fumigant, permit atmospheric moisture to activate the fumigant and permit the toxic gas to escape from the container and work its way into and through the grain. They also contain the powder residue, and can be separated from the grain at the destination, thereby satisfying those who are concerned about the presence of such residues in the grain.
Normal air movement in a body of grain in a hold is upward, so penetration of fumigating gas all the way to the bottom from fumigant that is placed at or near the top of the grain is questionable, particularly if the hold is on the order of sixty feet deep, as many are. Moreover, the sealing of the hatches of the holds is not always perfect, and gas generated at the top sometimes escapes from the top without effecting full penetration. Thus, fumigating from the top is not completely satisfactory.
Although it would be desirable to place the fumigant in the bottom of a hold before the grain is loaded, regulations prevent this because of the danger to the human beings who work in the hold as it is loaded. Efforts have been made to provide for the introduction of fumigating gas beneath the surface of grain, for example, as in the apertured pipe shown in U.S. Pat. No. 4,059,048, but have not been completely successful, probably because a "chimney" effect results in the release of most of the gas at or near the upper end.
The principal objections to the prior methods of placing fumigants in large storage compartments thus may be summarized as follows: the possibility of incomplete distribution of fumigating gas through the grain, and particularly downward from fumigant applied at or near the top; the time required in "probing" the fumigant into the upper portion of the grain; the objections of longshoremen to working in a hold after the fumigant is placed in the grain; the danger of loss is fumigating gas through imperfect seals before it has penetrated the grain; and the objections of some people to the mixing of powder residues with the grain. The primary object of this invention is to provide a method and apparatus for avoiding these objections, making it possible to fumigate large bodies of grain and other commodities more easily and effectively, and perhaps more safely, than has been possible with prior methods and apparatus.